A camshaft must meet various requirements; it must have good flexural strength, torsional strength and fatigue strength, but in particular the cam must be hard-wearing. The counter-part in direct contact with the cam is differently designed depending on the engine. The pinion partners used today for the cam are tappets, drag levers and valve lifters, in which sliding friction occurs, and roller tappets and roller lifters in which a rolling movement occurs.
The high forces which occur in the drive system and which act on the pinion partners exert various pressures on the contact surfaces of the pinion partners, which may cause damage in the case of an unsatisfactory design of the pinion partners, in particular at their surface (pitting, abrasion, etc.).
In systems where there is a sliding movement between the pinion partners in addition to high pressure, it must be ensured that the sliding surfaces of the cam additionally have an adequate wear-resistant layer to prevent abrasion of the cam.
The cast iron or clear chill cast iron camshafts used today are designed as one piece. These components, which in principle are proven ones, have the disadvantage that there are high processing costs, in particular in the operations such as cam grinding, thermal treatment, alignment, etc. and high spoilage rates in production from the blank to the ready-to-install camshaft. Another disadvantage is the high weight of the camshaft designed as a solid shaft. Modern surface coating processes for reducing wear are unsatisfactory because of the price and for technological reasons.
Patent publications have disclosed that attempts have been made to replace the present one-piece cast iron or clear chill cast iron camshafts with a multi-piece steel camshaft. This version was intended in particular to achieve a weight reduction through the use of a hollow shaft (pipe). In certain constructions, lower production costs were also expected.
All known multi-piece camshafts have in common the fact that the cast blank of the one-piece camshaft is replaced with a multi-piece steel camshaft blank. In the case of the production costs, a certain saving is achieved in the machining costs. The expensive finishing of the end position, the bearing seat and cams is, however, still part of the existing production technology. Here too, surface coating processes for improving wear are unsuccessful for technological reasons (size, selective area).
All joining methods disclosed, i.e. the joining of the cam to the shaft without torsion and axial displacement, have the disadvantage that, owing to the joining technology chosen, the required final geometric state of the mounted camshaft is not achieved without further machining.
The prior publication DE 3717534 A1 describes a constructed camshaft in which a relatively uncontrolled pressure build-up takes place in the cam as a result of widening the hollow shaft from inside--as found in practice--and acts as far as the cam surface by transmission via the points adjacent to the shaft, and irregularities, for example central indentations, may form there. This important disadvantage on the one hand prevents the use of cams machined to final dimensions and on the other hand prevents the use of novel production and surface coating processes.
A possible method for producing the cams and hollow shaft to final dimensions, including wear layer, prior to assembly would permit a considerable reduction in the unit costs compared with the conventional cast iron camshafts and also compared with the multi-piece assembled camshafts.
A connection must ensure that, in addition to a secure attachment for transmission of the resulting torsional and axial forces, the components manufactured to final dimensions, in particular cams and shaft, do not, as a result of joining, suffer any dimensional changes which impairs their function. The aim is to ensure that the camshaft is ready to install.